Glycosylation constitutes one of the most important of all post-translational protein modifications in eukaryotic cells and may have numerous effects on function, structure, physical properties and targeting of particular proteins. Generally, the carbohydrate moiety is to be regarded as having significant effects on both the structure and on the physicochemical features of a protein and may affect its enzymatic activity, antigenicity or thermal stability. The sugars can be linked via the ε-amine group of an asparagine (N-glycosidic bond) or the hydroxyl group of a serine or threonine (O-glycosidic bond) residue.
The N-linked protein glycosylation Is by far the most common protein modification found in eukaryotes. The complex glycosylation process starts at the cytoplasmic face of the endoplasmatic reticulum (ER) with the assembly of an oligosaccharide on the lipid carrier dolichylpyrophosphate [Burda, P. and Aebi, M. (1999) The dolichol pathway of N-linked glycosylation. Biochim Biophys Acta, 1426, 239-257]: 2 N-acetylglucosamine and 5 mannose residues are attached to this lipid in a stepwise fashion. The lipid linked oligosaccharide (LLO) is then flipped into the lumen of the ER, where by addition of 4 mannose and 3 glucose residues full length LLO is obtained. In the central reaction of the process, this oligosaccharide is transferred to selected asparagine residues of newly synthesized polypeptides. This reaction is catalyzed by the oligosaccharyl transferase (OTase) in the lumen of the ER. The OTase Is a complex of at least 8 subunits and this enzyme is responsible for the formation of the N-glycosidic bond. While still in the ER, three glucose and one mannose residue are quickly removed from the oligosaccharide of the glycoprotein. Glycoproteins are then transported to the Golgi apparatus where further trimming and addition of sugar moieties occurs before they are targeted to their final destinations [Varki, A., Cummings, R., Esko, J., Freeze, H., Hart, G. and Marth, 3. (1999) Essentials of Glycobiology. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.]. Whereas the LLO synthesis is a highly conserved process in eukaryotic cells, the modifications in the Golgi are not only species specific but also cell-type specific and lead to a high degree of diversity with respect to the structure of the N-linked oligosaccharides.